The subject invention relates to a process for the adsorptive separation of hydrocarbons. More specifically, the invention relates to a process for the continuous simulated countercurrent adsorptive separation of monomethyl paraffins from a mixture containing other hydrocarbons having the same number of carbon atoms per molecule. A preferred application of the process is the separation of C10-C15 monomethyl paraffins from a n-paraffin depleted kerosene boiling range fraction.
Most of the detergents in use today are derived from precursor petrochemicals. The currently predominant precursor is linear alkyl benzene (LAB), which is commonly produced by the alkylation of benzene with a long straight (normal) chain linear olefin. The subject invention is directed to the production of monomethyl acyclic olefins and paraffins, which may be recovered as a product in their own right, or used in the production of various petrochemicals as through alkylation or oxygenation. The following description of the invention will mainly address the recovery and use of the monomethyl hydrocarbons in the production of detergent precursor petrochemicals, and in particular the production of alkylbenzene derived detergents.
Several quality characteristics of alkylbenzenesulfonate (ABS) detergents are set by the chemical structure of the alkyl side chain. For instance, linear alkyl groups have the advantage of increased biodegradability. Other characteristics of a detergent such as its effectiveness in hard or cold water and its foaming tendency are also influenced by the structure of the side chain and its constituents. It has recently been determined that highly desirable detergent precursors can be formed from olefins which contain on average approximately one methyl side chain on the main alkane chain. These have been termed xe2x80x9cslightlyxe2x80x9d branched paraffins. Alkylbenzenes containing these monomethyl sidechains can be used by themselves or in admixture with linear alkyl benzenes to form a variety of detergent and cleaning products having superior cold and hard water properties. This is a departure from the previous preference for straight side chains. This unexpected advantage of monomethyl alkylbenzenes as detergent ingredients is described in U.S. Pat. Nos. 6,232,282 B1 and 6,228,829 B1. The subject invention is specifically directed to the production of monomethyl hydrocarbons for use in the subsequent production of the detergents and cleaning products of these two patents.
The large utility of detergents and other cleaners has led to extensive development in the areas of detergent production and formulation. While detergents can be formulated from a wide variety of different compounds much of the world""s supply is formulated from chemicals derived from alkyl benzenes. The compounds are produced in petrochemical complexes in which an aromatic hydrocarbon, typically benzene, is alkylated with an olefin of the desired structure and carbon number for the side chain. Typically the olefin is actually a mixture of different olefins forming a homologous series having a range of three to five carbon numbers. The olefin(s) can be derived from several alternative sources. For instance, they can be derived from the oligomerization of C3 or C4 olefins or from the polymerization of ethylene. Economics has led to the production of olefins by the dehydrogenation of the corresponding paraffin being the preferred route to produce the olefin.
Paraffins having 8 to 15 carbon atoms per molecule are present in significant concentrations in relatively low cost kerosene boiling range fractions of crude oils or processed fractions of crude oil. The carbon number range is set by the boiling point range of the kerosene. Recovery of the desired paraffins from kerosene by adsorption has become the leading commercial source of the olefinic precursors. The production of the olefins starts with recovery of paraffins of the same carbon number by adsorptive separation from kerosene. The paraffins are then passed through a catalytic dehydrogenation zone wherein some of the paraffins are converted to olefins. The resultant mixture of paraffins and olefins is then passed into an alkylation zone in which the olefins are reacted with the aromatic substrate. This overall flow is shown in U.S. Pat. No. 5,276,231 directed to an improvement related to the adsorptive separation of byproduct aromatic hydrocarbons from the dehydrogenation zone effluent. PCT International Publication WO 99/07656 indicates that paraffins used in this overall process may be recovered through the use of two adsorptive separation zones in series, with one zone producing normal paraffins and another producing mono-methyl paraffins.
A description of the use of simulated moving bed adsorptive separation to recover paraffins from a kerosene boiling range petroleum fraction is provided in a presentation made by R. C. Shulz et al. at the 2nd World Conference on Detergents in Montreux, Switzerland on Oct. 5-10, 1986. This shows several incidental steps in the process such as fractionation and hydrotreating.
The success of a particular adsorptive separation is determined by many factors. Predominant among these are the composition of the adsorbent (stationary phase) and desorbent (mobile phase) employed in the process. The remaining factors are basically related to process conditions, which are very important to successful commercial operation. The subject process employs an adsorbent comprising a molecular sieve referred to in the art as silicalite. The use of silicalite in the adsorptive separation of paraffins is described in U.S. Pat. No. 4,956,521 issued to W. K. Volles, which is directed to the production of higher octane gasoline blending components. The sequential use of silicalite and zeolite 5A in the separation of monomethylalkanes is described in an article in the Joumal of Chromatography, 316 (1984) 333-341. Silicalite has also been described as useful in separating normal paraffins from cyclic hydrocarbons and from branched chain hydrocarbons in U.S. Pat. Nos. 4,367,364 and 4,455,444 issued to S. Kulprathipanja and R. W. Neuzil. This separation differs from that performed in the subject process as it corresponds to that done in the previously cited article from the World Conference on Detergents, which is performed to recover normal paraffins.
The unique pore structure of silicalite has also led to efforts to employ it in the separation of linear (normal) olefins. However, silicalite also has catalytic properties which can result in undesired conversion of olefins during this separation. The use of silicalite based adsorbents in the separation of linear olefins from nonlinear hydrocarbons and treatments of the silicalite to reduce its catalytic activity are described in U.S. Pat. Nos. 5,262,144 to McCulloch; 5,276,246 to McCulloch et al, and 5,292,990 to Kanter et al.
Temperature has been recognized to be important operating parameter in SMB processes. Temperature ranges for traditional normal paraffin SMB processes are set out in U.S. Pat. Nos. 4,367,364 and 4,992,618. The latter also mentions a cycle time of 60 minutes.
The invention is a simulated moving bed adsorptive separation process for the recovery of monomethyl paraffins or olefins from admixture with other nonnormal paraffins or olefins, e.g., cyclic and multibranched paraffins of the same carbon number. The invention is characterized by the use of a unique set of operating conditions including low adsorption zone cycle time and temperature.
A broad embodiment of the invention may be characterized as a simulated moving bed adsorptive separation process for the separation of a C8 to C14 monomethyl paraffin from a feed mixture comprising the monomethyl paraffin and at least one other acyclic C8 to C14 non-normal hydrocarbon of the same carbon number, with the feed mixture containing less than 5 wt. % normal C8 to C14 paraffins, which process comprises passing the feed mixture into a bed of adsorbent under conditions which result in the removal of less than 95 wt. percent of the monomethyl paraffin from the feed mixture and include an A/F ratio of 0.5 to 1.5, a temperature of from about 30 to 120xc2x0 C., and a cycle time of about 20 to 60 minutes, and then recovering selectively adsorbed monomethyl paraffin from the bed of adsorbent by contacting the bed of adsorbent with a desorbent compound. While the removal of the monomethyl paraffin is intentionally limited, it is a positive amount preferably greater than 50 percent and more preferably greater than 75 percent. A preferred monomethyl recovery range is from 80-95 percent. A more preferred cycle time range is from 20 to 45 minutes.